1 files changed, 848 insertions, 0 deletions
diff --git a/js/src/vm/Xdr.cpp b/js/src/vm/Xdr.cpp
new file mode 100644
index 0000000000..75ccc264a1
--- /dev/null
+++ b/js/src/vm/Xdr.cpp
@@ -0,0 +1,848 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "vm/Xdr.h"
+
+#include "mozilla/ArrayUtils.h"
+#include "mozilla/ScopeExit.h"
+#include "mozilla/Utf8.h"
+
+#include <algorithm>  // std::transform
+#include <string.h>
+#include <type_traits>  // std::is_same
+#include <utility>      // std::move
+
+#include "jsapi.h"
+
+#include "builtin/ModuleObject.h"
+#include "debugger/DebugAPI.h"
+#include "frontend/CompilationInfo.h"  // frontend::BaseCompilationStencil, frontend::CompilationStencil, frontend::CompilationStencilSet
+#include "frontend/ParserAtom.h"       // frontend::ParserAtomEntry
+#include "js/BuildId.h"                // JS::BuildIdCharVector
+#include "vm/JSContext.h"
+#include "vm/JSScript.h"
+#include "vm/SharedStencil.h"  // js::SourceExtent
+#include "vm/TraceLogging.h"
+
+using namespace js;
+
+using mozilla::ArrayEqual;
+using mozilla::Utf8Unit;
+
+#ifdef DEBUG
+bool XDRCoderBase::validateResultCode(JSContext* cx,
+                                      JS::TranscodeResult code) const {
+  // NOTE: This function is called to verify that we do not have a pending
+  // exception on the JSContext at the same time as a TranscodeResult failure.
+  if (cx->isHelperThreadContext()) {
+    return true;
+  }
+  return cx->isExceptionPending() == bool(code == JS::TranscodeResult_Throw);
+}
+#endif
+
+template <XDRMode mode>
+XDRResult XDRState<mode>::codeChars(char* chars, size_t nchars) {
+  return codeBytes(chars, nchars);
+}
+
+template <XDRMode mode>
+XDRResult XDRState<mode>::codeChars(Latin1Char* chars, size_t nchars) {
+  static_assert(sizeof(Latin1Char) == 1,
+                "Latin1Char must be 1 byte for nchars below to be the "
+                "proper count of bytes");
+  static_assert(std::is_same_v<Latin1Char, unsigned char>,
+                "Latin1Char must be unsigned char to C++-safely reinterpret "
+                "the bytes generically copied below as Latin1Char");
+  return codeBytes(chars, nchars);
+}
+
+template <XDRMode mode>
+XDRResult XDRState<mode>::codeChars(Utf8Unit* units, size_t count) {
+  if (count == 0) {
+    return Ok();
+  }
+
+  if (mode == XDR_ENCODE) {
+    uint8_t* ptr = buf->write(count);
+    if (!ptr) {
+      return fail(JS::TranscodeResult_Throw);
+    }
+
+    std::transform(units, units + count, ptr,
+                   [](const Utf8Unit& unit) { return unit.toUint8(); });
+  } else {
+    const uint8_t* ptr = buf->read(count);
+    if (!ptr) {
+      return fail(JS::TranscodeResult_Failure_BadDecode);
+    }
+
+    std::transform(ptr, ptr + count, units,
+                   [](const uint8_t& value) { return Utf8Unit(value); });
+  }
+
+  return Ok();
+}
+
+template <XDRMode mode>
+XDRResult XDRState<mode>::codeChars(char16_t* chars, size_t nchars) {
+  if (nchars == 0) {
+    return Ok();
+  }
+
+  size_t nbytes = nchars * sizeof(char16_t);
+  if (mode == XDR_ENCODE) {
+    uint8_t* ptr = buf->write(nbytes);
+    if (!ptr) {
+      return fail(JS::TranscodeResult_Throw);
+    }
+
+    // |mozilla::NativeEndian| correctly handles writing into unaligned |ptr|.
+    mozilla::NativeEndian::copyAndSwapToLittleEndian(ptr, chars, nchars);
+  } else {
+    const uint8_t* ptr = buf->read(nbytes);
+    if (!ptr) {
+      return fail(JS::TranscodeResult_Failure_BadDecode);
+    }
+
+    // |mozilla::NativeEndian| correctly handles reading from unaligned |ptr|.
+    mozilla::NativeEndian::copyAndSwapFromLittleEndian(chars, ptr, nchars);
+  }
+  return Ok();
+}
+
+template <XDRMode mode, typename CharT>
+static XDRResult XDRCodeCharsZ(XDRState<mode>* xdr,
+                               XDRTranscodeString<CharT>& buffer) {
+  MOZ_ASSERT_IF(mode == XDR_ENCODE, !buffer.empty());
+  MOZ_ASSERT_IF(mode == XDR_DECODE, buffer.empty());
+
+  using OwnedString = js::UniquePtr<CharT[], JS::FreePolicy>;
+  OwnedString owned;
+
+  static_assert(JSString::MAX_LENGTH <= INT32_MAX,
+                "String length must fit in int32_t");
+
+  uint32_t length = 0;
+  CharT* chars = nullptr;
+
+  if (mode == XDR_ENCODE) {
+    chars = const_cast<CharT*>(buffer.template ref<const CharT*>());
+
+    // Set a reasonable limit on string length.
+    size_t lengthSizeT = std::char_traits<CharT>::length(chars);
+    if (lengthSizeT > JSString::MAX_LENGTH) {
+      ReportAllocationOverflow(xdr->cx());
+      return xdr->fail(JS::TranscodeResult_Throw);
+    }
+    length = static_cast<uint32_t>(lengthSizeT);
+  }
+  MOZ_TRY(xdr->codeUint32(&length));
+
+  if (mode == XDR_DECODE) {
+    owned = xdr->cx()->template make_pod_array<CharT>(length + 1);
+    if (!owned) {
+      return xdr->fail(JS::TranscodeResult_Throw);
+    }
+    chars = owned.get();
+  }
+
+  MOZ_TRY(xdr->codeChars(chars, length));
+  if (mode == XDR_DECODE) {
+    // Null-terminate and transfer ownership to caller.
+    owned[length] = '\0';
+    buffer.template construct<OwnedString>(std::move(owned));
+  }
+
+  return Ok();
+}
+
+template <XDRMode mode>
+XDRResult XDRState<mode>::codeCharsZ(XDRTranscodeString<char>& buffer) {
+  return XDRCodeCharsZ(this, buffer);
+}
+
+template <XDRMode mode>
+XDRResult XDRState<mode>::codeCharsZ(XDRTranscodeString<char16_t>& buffer) {
+  return XDRCodeCharsZ(this, buffer);
+}
+
+enum class XDRFormatType : uint8_t {
+  UseOption,
+  JSScript,
+  Stencil,
+};
+
+static bool GetScriptTranscodingBuildId(XDRFormatType formatType,
+                                        JS::BuildIdCharVector* buildId) {
+  MOZ_ASSERT(buildId->empty());
+  MOZ_ASSERT(GetBuildId);
+
+  if (!GetBuildId(buildId)) {
+    return false;
+  }
+
+  // Note: the buildId returned here is also used for the bytecode cache MIME
+  // type so use plain ASCII characters.
+
+  if (!buildId->reserve(buildId->length() + 4)) {
+    return false;
+  }
+
+  buildId->infallibleAppend('-');
+
+  // XDR depends on pointer size and endianness.
+  static_assert(sizeof(uintptr_t) == 4 || sizeof(uintptr_t) == 8);
+  buildId->infallibleAppend(sizeof(uintptr_t) == 4 ? '4' : '8');
+  buildId->infallibleAppend(MOZ_LITTLE_ENDIAN() ? 'l' : 'b');
+
+  // '0': Stencil
+  // '1': JSScript.
+  char formatChar = '0';
+  switch (formatType) {
+    case XDRFormatType::UseOption:
+      // If off-thread parse global isn't used for single script decoding,
+      // we use stencil XDR instead of JSScript XDR.
+      formatChar = js::UseOffThreadParseGlobal() ? '1' : '0';
+      break;
+    case XDRFormatType::JSScript:
+      formatChar = '1';
+      break;
+    case XDRFormatType::Stencil:
+      formatChar = '0';
+      break;
+  }
+  buildId->infallibleAppend(formatChar);
+
+  return true;
+}
+
+JS_PUBLIC_API bool JS::GetScriptTranscodingBuildId(
+    JS::BuildIdCharVector* buildId) {
+  return GetScriptTranscodingBuildId(XDRFormatType::UseOption, buildId);
+}
+
+template <XDRMode mode>
+static XDRResult VersionCheck(XDRState<mode>* xdr, XDRFormatType formatType) {
+  JS::BuildIdCharVector buildId;
+  if (!GetScriptTranscodingBuildId(formatType, &buildId)) {
+    ReportOutOfMemory(xdr->cx());
+    return xdr->fail(JS::TranscodeResult_Throw);
+  }
+  MOZ_ASSERT(!buildId.empty());
+
+  uint32_t buildIdLength;
+  if (mode == XDR_ENCODE) {
+    buildIdLength = buildId.length();
+  }
+
+  MOZ_TRY(xdr->codeUint32(&buildIdLength));
+
+  if (mode == XDR_DECODE && buildIdLength != buildId.length()) {
+    return xdr->fail(JS::TranscodeResult_Failure_BadBuildId);
+  }
+
+  if (mode == XDR_ENCODE) {
+    MOZ_TRY(xdr->codeBytes(buildId.begin(), buildIdLength));
+  } else {
+    JS::BuildIdCharVector decodedBuildId;
+
+    // buildIdLength is already checked against the length of current
+    // buildId.
+    if (!decodedBuildId.resize(buildIdLength)) {
+      ReportOutOfMemory(xdr->cx());
+      return xdr->fail(JS::TranscodeResult_Throw);
+    }
+
+    MOZ_TRY(xdr->codeBytes(decodedBuildId.begin(), buildIdLength));
+
+    // We do not provide binary compatibility with older scripts.
+    if (!ArrayEqual(decodedBuildId.begin(), buildId.begin(), buildIdLength)) {
+      return xdr->fail(JS::TranscodeResult_Failure_BadBuildId);
+    }
+  }
+
+  return Ok();
+}
+
+template <XDRMode mode>
+XDRResult XDRState<mode>::codeModuleObject(MutableHandleModuleObject modp) {
+#ifdef DEBUG
+  auto sanityCheck = mozilla::MakeScopeExit(
+      [&] { MOZ_ASSERT(validateResultCode(cx(), resultCode())); });
+#endif
+  if (mode == XDR_DECODE) {
+    modp.set(nullptr);
+  } else {
+    MOZ_ASSERT(modp->status() < MODULE_STATUS_LINKING);
+  }
+
+  MOZ_TRY(XDRModuleObject(this, modp));
+  return Ok();
+}
+
+template <XDRMode mode>
+static XDRResult XDRAtomCount(XDRState<mode>* xdr, uint32_t* atomCount) {
+  return xdr->codeUint32(atomCount);
+}
+
+template <XDRMode mode>
+static XDRResult AtomTable(XDRState<mode>* xdr) {
+  uint8_t atomHeader = false;
+  if (mode == XDR_ENCODE) {
+    if (xdr->hasAtomMap()) {
+      atomHeader = true;
+    }
+  }
+
+  MOZ_TRY(xdr->codeUint8(&atomHeader));
+
+  // If we are incrementally encoding, the atom table will be built up over the
+  // course of the encoding. In XDRIncrementalEncoder::linearize, we will write
+  // the number of atoms into the header, then append the completed atom table.
+  // If we are decoding, then we read the length and decode the atom table now.
+  if (atomHeader && mode == XDR_DECODE) {
+    uint32_t atomCount;
+    MOZ_TRY(XDRAtomCount(xdr, &atomCount));
+    MOZ_ASSERT(!xdr->hasAtomTable());
+
+    for (uint32_t i = 0; i < atomCount; i++) {
+      RootedAtom atom(xdr->cx());
+      MOZ_TRY(XDRAtom(xdr, &atom));
+      if (!xdr->atomTable().append(atom)) {
+        return xdr->fail(JS::TranscodeResult_Throw);
+      }
+    }
+    xdr->finishAtomTable();
+  }
+
+  return Ok();
+}
+
+template <XDRMode mode>
+static XDRResult XDRParserAtomTable(XDRState<mode>* xdr,
+                                    frontend::BaseCompilationStencil& stencil) {
+  if (mode == XDR_ENCODE) {
+    uint32_t atomVectorLength = stencil.parserAtomData.size();
+    MOZ_TRY(XDRAtomCount(xdr, &atomVectorLength));
+
+    uint32_t atomCount = 0;
+    for (const auto& entry : stencil.parserAtomData) {
+      if (!entry) {
+        continue;
+      }
+      if (entry->isUsedByStencil()) {
+        atomCount++;
+      }
+    }
+    MOZ_TRY(XDRAtomCount(xdr, &atomCount));
+
+    for (auto& entry : stencil.parserAtomData) {
+      if (!entry) {
+        continue;
+      }
+      if (entry->isUsedByStencil()) {
+        uint32_t index = entry->toParserAtomIndex();
+        MOZ_TRY(xdr->codeUint32(&index));
+        MOZ_TRY(XDRParserAtomEntry(xdr, &entry));
+      }
+    }
+
+    return Ok();
+  }
+
+  uint32_t atomVectorLength;
+  MOZ_TRY(XDRAtomCount(xdr, &atomVectorLength));
+
+  if (!xdr->frontendAtoms().allocate(xdr->cx(), xdr->stencilAlloc(),
+                                     atomVectorLength)) {
+    return xdr->fail(JS::TranscodeResult_Throw);
+  }
+
+  uint32_t atomCount;
+  MOZ_TRY(XDRAtomCount(xdr, &atomCount));
+  MOZ_ASSERT(!xdr->hasAtomTable());
+
+  for (uint32_t i = 0; i < atomCount; i++) {
+    frontend::ParserAtomEntry* entry = nullptr;
+    uint32_t index;
+    MOZ_TRY(xdr->codeUint32(&index));
+    MOZ_TRY(XDRParserAtomEntry(xdr, &entry));
+    xdr->frontendAtoms().set(frontend::ParserAtomIndex(index), entry);
+  }
+  xdr->finishAtomTable();
+
+  return Ok();
+}
+
+template <XDRMode mode>
+static XDRResult XDRChunkCount(XDRState<mode>* xdr, uint32_t* sliceCount) {
+  return xdr->codeUint32(sliceCount);
+}
+
+template <XDRMode mode>
+XDRResult XDRState<mode>::codeFunction(MutableHandleFunction funp,
+                                       HandleScriptSourceObject sourceObject) {
+  TraceLoggerThread* logger = TraceLoggerForCurrentThread(cx());
+  TraceLoggerTextId event = mode == XDR_DECODE ? TraceLogger_DecodeFunction
+                                               : TraceLogger_EncodeFunction;
+  AutoTraceLog tl(logger, event);
+
+#ifdef DEBUG
+  auto sanityCheck = mozilla::MakeScopeExit(
+      [&] { MOZ_ASSERT(validateResultCode(cx(), resultCode())); });
+#endif
+  auto guard = mozilla::MakeScopeExit([&] { funp.set(nullptr); });
+  RootedScope scope(cx(), &cx()->global()->emptyGlobalScope());
+  if (mode == XDR_DECODE) {
+    MOZ_ASSERT(!sourceObject);
+    funp.set(nullptr);
+  } else if (getTreeKey(funp) != AutoXDRTree::noKey) {
+    MOZ_ASSERT(sourceObject);
+    scope = funp->enclosingScope();
+  } else {
+    MOZ_ASSERT(!sourceObject);
+    MOZ_ASSERT(funp->enclosingScope()->is<GlobalScope>());
+  }
+
+  MOZ_TRY(VersionCheck(this, XDRFormatType::JSScript));
+  MOZ_TRY(XDRInterpretedFunction(this, scope, sourceObject, funp));
+
+  guard.release();
+  return Ok();
+}
+
+template <XDRMode mode>
+XDRResult XDRState<mode>::codeScript(MutableHandleScript scriptp) {
+  TraceLoggerThread* logger = TraceLoggerForCurrentThread(cx());
+  TraceLoggerTextId event =
+      mode == XDR_DECODE ? TraceLogger_DecodeScript : TraceLogger_EncodeScript;
+  AutoTraceLog tl(logger, event);
+
+#ifdef DEBUG
+  auto sanityCheck = mozilla::MakeScopeExit(
+      [&] { MOZ_ASSERT(validateResultCode(cx(), resultCode())); });
+#endif
+  auto guard = mozilla::MakeScopeExit([&] { scriptp.set(nullptr); });
+
+  AutoXDRTree scriptTree(this, getTopLevelTreeKey());
+
+  if (mode == XDR_DECODE) {
+    scriptp.set(nullptr);
+  } else {
+    MOZ_ASSERT(!scriptp->enclosingScope());
+  }
+
+  // Only write to separate header buffer if we are incrementally encoding.
+  bool useHeader = this->hasAtomMap();
+  if (useHeader) {
+    switchToHeaderBuf();
+  }
+  MOZ_TRY(VersionCheck(this, XDRFormatType::JSScript));
+  MOZ_TRY(AtomTable(this));
+  if (useHeader) {
+    switchToMainBuf();
+  }
+  MOZ_ASSERT(isMainBuf());
+  MOZ_TRY(XDRScript(this, nullptr, nullptr, nullptr, scriptp));
+
+  guard.release();
+  return Ok();
+}
+
+template <XDRMode mode>
+static XDRResult XDRStencilHeader(
+    XDRState<mode>* xdr, const JS::ReadOnlyCompileOptions* maybeOptions,
+    MutableHandle<ScriptSourceHolder> source, uint32_t* pNumChunks) {
+  // The XDR-Stencil header is inserted at beginning of buffer, but it is
+  // computed at the end the incremental-encoding process.
+
+  MOZ_TRY(VersionCheck(xdr, XDRFormatType::Stencil));
+  MOZ_TRY(ScriptSource::XDR(xdr, maybeOptions, source));
+  MOZ_TRY(XDRChunkCount(xdr, pNumChunks));
+  MOZ_TRY(xdr->align32());
+
+  return Ok();
+}
+
+template <XDRMode mode>
+XDRResult XDRState<mode>::codeStencil(frontend::CompilationStencil& stencil) {
+#ifdef DEBUG
+  auto sanityCheck = mozilla::MakeScopeExit(
+      [&] { MOZ_ASSERT(validateResultCode(cx(), resultCode())); });
+#endif
+
+  // Instrumented scripts cannot be encoded, as they have extra instructions
+  // which are not normally present. Globals with instrumentation enabled must
+  // compile scripts via the bytecode emitter, which will insert these
+  // instructions.
+  if (mode == XDR_ENCODE) {
+    if (!!stencil.input.options.instrumentationKinds) {
+      return fail(JS::TranscodeResult_Failure);
+    }
+  }
+
+  // Process the header now if decoding. If we are encoding, we defer generating
+  // the header data until the `linearize` call, but still prepend it to final
+  // buffer before giving to the caller.
+  if (mode == XDR_DECODE) {
+    Rooted<ScriptSourceHolder> holder(cx());
+    MOZ_TRY(
+        XDRStencilHeader(this, &stencil.input.options, &holder, &nchunks()));
+    stencil.input.setSource(holder.get().get());
+  }
+
+  MOZ_ASSERT(isMainBuf());
+
+  MOZ_TRY(XDRParserAtomTable(this, stencil));
+  MOZ_TRY(XDRCompilationStencil(this, stencil));
+
+  return Ok();
+}
+
+template <XDRMode mode>
+XDRResult XDRState<mode>::codeFunctionStencil(
+    frontend::BaseCompilationStencil& stencil) {
+#ifdef DEBUG
+  auto sanityCheck = mozilla::MakeScopeExit(
+      [&] { MOZ_ASSERT(validateResultCode(cx(), resultCode())); });
+#endif
+  bool isAlreadyCoded = false;
+  MOZ_TRY_VAR(isAlreadyCoded, checkAlreadyCoded(stencil));
+  if (isAlreadyCoded) {
+    return Ok();
+  }
+
+  MOZ_TRY(XDRParserAtomTable(this, stencil));
+  MOZ_TRY(XDRBaseCompilationStencil(this, stencil));
+
+  return Ok();
+}
+
+template class js::XDRState<XDR_ENCODE>;
+template class js::XDRState<XDR_DECODE>;
+
+AutoXDRTree::AutoXDRTree(XDRCoderBase* xdr, AutoXDRTree::Key key)
+    : key_(key), parent_(this), xdr_(xdr) {
+  if (key_ != AutoXDRTree::noKey) {
+    xdr->createOrReplaceSubTree(this);
+  }
+}
+
+AutoXDRTree::~AutoXDRTree() {
+  if (key_ != AutoXDRTree::noKey) {
+    xdr_->endSubTree();
+  }
+}
+
+constexpr AutoXDRTree::Key AutoXDRTree::noKey;
+constexpr AutoXDRTree::Key AutoXDRTree::noSubTree;
+constexpr AutoXDRTree::Key AutoXDRTree::topLevel;
+
+class XDRIncrementalEncoder::DepthFirstSliceIterator {
+ public:
+  DepthFirstSliceIterator(JSContext* cx, const SlicesTree& tree)
+      : stack_(cx), tree_(tree) {}
+
+  template <typename SliceFun>
+  bool iterate(SliceFun&& f) {
+    MOZ_ASSERT(stack_.empty());
+
+    if (!appendChildrenForKey(AutoXDRTree::topLevel)) {
+      return false;
+    }
+
+    while (!done()) {
+      SlicesNode::ConstRange& iter = next();
+      Slice slice = iter.popCopyFront();
+      // These fields have different meaning, but they should be
+      // correlated if the tree is well formatted.
+      MOZ_ASSERT_IF(slice.child == AutoXDRTree::noSubTree, iter.empty());
+      if (iter.empty()) {
+        pop();
+      }
+
+      if (!f(slice)) {
+        return false;
+      }
+
+      // If we are at the end, go back to the parent script.
+      if (slice.child == AutoXDRTree::noSubTree) {
+        continue;
+      }
+
+      if (!appendChildrenForKey(slice.child)) {
+        return false;
+      }
+    }
+
+    return true;
+  }
+
+ private:
+  bool done() const { return stack_.empty(); }
+  SlicesNode::ConstRange& next() { return stack_.back(); }
+  void pop() { stack_.popBack(); }
+
+  MOZ_MUST_USE bool appendChildrenForKey(AutoXDRTree::Key key) {
+    MOZ_ASSERT(key != AutoXDRTree::noSubTree);
+
+    SlicesTree::Ptr p = tree_.lookup(key);
+    MOZ_ASSERT(p);
+    return stack_.append(((const SlicesNode&)p->value()).all());
+  }
+
+  Vector<SlicesNode::ConstRange> stack_;
+  const SlicesTree& tree_;
+};
+
+AutoXDRTree::Key XDRIncrementalEncoder::getTopLevelTreeKey() const {
+  return AutoXDRTree::topLevel;
+}
+
+AutoXDRTree::Key XDRIncrementalEncoder::getTreeKey(JSFunction* fun) const {
+  if (fun->hasBaseScript()) {
+    static_assert(sizeof(fun->baseScript()->sourceStart()) == 4 &&
+                      sizeof(fun->baseScript()->sourceEnd()) == 4,
+                  "AutoXDRTree key requires BaseScript positions to be uint32");
+    return uint64_t(fun->baseScript()->sourceStart()) << 32 |
+           fun->baseScript()->sourceEnd();
+  }
+
+  return AutoXDRTree::noKey;
+}
+
+void XDRIncrementalEncoder::createOrReplaceSubTree(AutoXDRTree* child) {
+  AutoXDRTree* parent = scope_;
+  child->parent_ = parent;
+  scope_ = child;
+  if (oom_) {
+    return;
+  }
+
+  size_t cursor = buf->cursor();
+
+  // End the parent slice here, set the key to the child.
+  if (parent) {
+    Slice& last = node_->back();
+    last.sliceLength = cursor - last.sliceBegin;
+    last.child = child->key_;
+    MOZ_ASSERT_IF(uint32_t(parent->key_) != 0,
+                  uint32_t(parent->key_ >> 32) <= uint32_t(child->key_ >> 32) &&
+                      uint32_t(child->key_) <= uint32_t(parent->key_));
+  }
+
+  // Create or replace the part with what is going to be encoded next.
+  SlicesTree::AddPtr p = tree_.lookupForAdd(child->key_);
+  SlicesNode tmp;
+  if (!p) {
+    // Create a new sub-tree node.
+    if (!tree_.add(p, child->key_, std::move(tmp))) {
+      oom_ = true;
+      return;
+    }
+  } else {
+    // Replace an exisiting sub-tree.
+    p->value() = std::move(tmp);
+  }
+  node_ = &p->value();
+
+  // Add content to the root of the new sub-tree,
+  // i-e an empty slice with no children.
+  if (!node_->append(Slice{cursor, 0, AutoXDRTree::noSubTree})) {
+    MOZ_CRASH("SlicesNode have a reserved space of 1.");
+  }
+}
+
+void XDRIncrementalEncoder::endSubTree() {
+  AutoXDRTree* child = scope_;
+  AutoXDRTree* parent = child->parent_;
+  scope_ = parent;
+  if (oom_) {
+    return;
+  }
+
+  size_t cursor = buf->cursor();
+
+  // End the child sub-tree.
+  Slice& last = node_->back();
+  last.sliceLength = cursor - last.sliceBegin;
+  MOZ_ASSERT(last.child == AutoXDRTree::noSubTree);
+
+  // Stop at the top-level.
+  if (!parent) {
+    node_ = nullptr;
+    return;
+  }
+
+  // Restore the parent node.
+  SlicesTree::Ptr p = tree_.lookup(parent->key_);
+  node_ = &p->value();
+
+  // Append the new slice in the parent node.
+  if (!node_->append(Slice{cursor, 0, AutoXDRTree::noSubTree})) {
+    oom_ = true;
+    return;
+  }
+}
+
+XDRResult XDRIncrementalEncoder::linearize(JS::TranscodeBuffer& buffer,
+                                           ScriptSource* ss) {
+  if (oom_) {
+    ReportOutOfMemory(cx());
+    return fail(JS::TranscodeResult_Throw);
+  }
+
+  // Do not linearize while we are currently adding bytes.
+  MOZ_ASSERT(scope_ == nullptr);
+
+  // Write the size of the atom buffer to the header.
+  switchToHeaderBuf();
+  MOZ_TRY(XDRAtomCount(this, &natoms_));
+  switchToMainBuf();
+
+  // Visit the tree parts in a depth first order to linearize the bits.
+  // Calculate the total length first so we don't incur repeated copying
+  // and zeroing of memory for large trees.
+  DepthFirstSliceIterator dfs(cx(), tree_);
+
+  size_t totalLength = buffer.length() + header_.length() + atoms_.length();
+  auto sliceCounter = [&](const Slice& slice) -> bool {
+    totalLength += slice.sliceLength;
+    return true;
+  };
+
+  if (!dfs.iterate(sliceCounter)) {
+    ReportOutOfMemory(cx());
+    return fail(JS::TranscodeResult_Throw);
+  };
+
+  if (!buffer.reserve(totalLength)) {
+    ReportOutOfMemory(cx());
+    return fail(JS::TranscodeResult_Throw);
+  }
+
+  buffer.infallibleAppend(header_.begin(), header_.length());
+  buffer.infallibleAppend(atoms_.begin(), atoms_.length());
+
+  auto sliceCopier = [&](const Slice& slice) -> bool {
+    // Copy the bytes associated with the current slice to the transcode
+    // buffer which would be serialized.
+    MOZ_ASSERT(slice.sliceBegin <= slices_.length());
+    MOZ_ASSERT(slice.sliceBegin + slice.sliceLength <= slices_.length());
+
+    buffer.infallibleAppend(slices_.begin() + slice.sliceBegin,
+                            slice.sliceLength);
+    return true;
+  };
+
+  if (!dfs.iterate(sliceCopier)) {
+    ReportOutOfMemory(cx());
+    return fail(JS::TranscodeResult_Throw);
+  }
+
+  tree_.clearAndCompact();
+  slices_.clearAndFree();
+  return Ok();
+}
+
+XDRResult XDRIncrementalStencilEncoder::linearize(JS::TranscodeBuffer& buffer,
+                                                  ScriptSource* ss) {
+  // NOTE: If buffer is empty, buffer.begin() doesn't point valid buffer.
+  MOZ_ASSERT_IF(!buffer.empty(),
+                JS::IsTranscodingBytecodeAligned(buffer.begin()));
+  MOZ_ASSERT(JS::IsTranscodingBytecodeOffsetAligned(buffer.length()));
+
+  // Use the output buffer directly. The caller may have already have data in
+  // the buffer so ensure we skip over it.
+  XDRBuffer<XDR_ENCODE> outputBuf(cx(), buffer, buffer.length());
+
+  // Code the header directly in the output buffer.
+  {
+    switchToBuffer(&outputBuf);
+
+    Rooted<ScriptSourceHolder> holder(cx(), ss);
+    uint32_t nchunks = 1 + encodedFunctions_.count();
+    MOZ_TRY(XDRStencilHeader(this, nullptr, &holder, &nchunks));
+
+    switchToMainBuf();
+  }
+
+  // The accumlated transcode data can now be copied to the output buffer.
+  if (!buffer.append(slices_.begin(), slices_.length())) {
+    return fail(JS::TranscodeResult_Throw);
+  }
+
+  return Ok();
+}
+
+void XDRDecoder::trace(JSTracer* trc) { atomTable_.trace(trc); }
+
+void XDRIncrementalEncoder::trace(JSTracer* trc) { atomMap_.trace(trc); }
+
+XDRResult XDRStencilDecoder::codeStencils(
+    frontend::CompilationStencilSet& stencilSet) {
+  MOZ_ASSERT(stencilSet.delazifications.length() == 0);
+
+  frontend::ParserAtomSpanBuilder parserAtomBuilder(cx()->runtime(),
+                                                    stencilSet.parserAtomData);
+  parserAtomBuilder_ = &parserAtomBuilder;
+  stencilAlloc_ = &stencilSet.alloc;
+
+  MOZ_TRY(codeStencil(stencilSet));
+
+  if (!stencilSet.delazifications.reserve(nchunks_ - 1)) {
+    ReportOutOfMemory(cx());
+    return fail(JS::TranscodeResult_Throw);
+  }
+
+  for (size_t i = 1; i < nchunks_; i++) {
+    stencilSet.delazifications.infallibleEmplaceBack();
+    auto& delazification = stencilSet.delazifications[i - 1];
+
+    hasFinishedAtomTable_ = false;
+
+    frontend::ParserAtomSpanBuilder parserAtomBuilder(
+        cx()->runtime(), delazification.parserAtomData);
+    parserAtomBuilder_ = &parserAtomBuilder;
+
+    MOZ_TRY(codeFunctionStencil(delazification));
+  }
+
+  return Ok();
+}
+
+XDRResult XDRIncrementalStencilEncoder::codeStencils(
+    frontend::CompilationStencilSet& stencilSet) {
+  MOZ_ASSERT(encodedFunctions_.count() == 0);
+
+  MOZ_TRY(codeStencil(stencilSet));
+
+  for (auto& delazification : stencilSet.delazifications) {
+    MOZ_TRY(codeFunctionStencil(delazification));
+  }
+
+  return Ok();
+}
+
+XDRResultT<bool> XDRIncrementalStencilEncoder::checkAlreadyCoded(
+    const frontend::BaseCompilationStencil& stencil) {
+  static_assert(std::is_same_v<frontend::BaseCompilationStencil::FunctionKey,
+                               XDRIncrementalStencilEncoder::FunctionKey>);
+
+  auto key = stencil.functionKey;
+  auto p = encodedFunctions_.lookupForAdd(key);
+  if (p) {
+    return true;
+  }
+
+  if (!encodedFunctions_.add(p, key)) {
+    ReportOutOfMemory(cx());
+    return fail<bool>(JS::TranscodeResult_Throw);
+  }
+
+  return false;
+}